Do good neighbours compensate for bad climate? The case of sessile oak Quercus petraea in phylogenetically distant neighbourhoods

Stressful climates threaten the performance of individuals and the survival of species, but individual performance also depends on biotic interactions with neighbours, so that ‘good' neighbours might in theory compensate for ‘bad' climates. However, what makes a good neighbour, and why, has been little studied. We hypothesize that phylogenetically proximate neighbours either interact positively (by sharing mutualists), thereby helping to resist climate stress, or they interact negatively (by sharing enemies or competing), thereby reducing resistance. We studied seed germination, juvenile growth, and adult phenology of sessile oaks Quercus petraea and how they respond to microclimatic stress in phylogenetically proximate and distant neighbourhoods, manipulating offspring‐density, the presence of insects and fungi, and spatial proximity to oaks. We found that juvenile oaks were more resistant to drought stress when growing in phylogenetically distant neighbourhoods. Also, the budburst delay in adults in phylogenetically distant neighbourhoods disappeared under drought. In contrast, under drought, seeds germinated better in the presence of conspecific adults. Most importantly, under humid or cold conditions, phylogenetically proximate neighbours were more favourable in multiple ways. Differences in resistance to microclimate‐stress among phylogenetically proximate and distant neighbourhoods reflect the activity of fungi, insects, or the interaction with conspecific adult neighbours, i.e. differences disappear once the pertinent factor is experimentally excluded. These interactive effects were mostly stronger than the main effects of microclimatic variables and phylogenetic distance. Effects of phylogenetic distance were partly replaceable by the percentage of gymnosperms. We suggest that under the increased temperature and decreased precipitation due to climate change, oak regeneration may profit from a phylogenetically distant neighbourhood, partly in the presence of a spatially proximate conspecific. At the same time, such warmer and drier conditions eliminate the advantages that oaks have from a phylogenetically proximate neighbourhood, potentially contributing to an erosion of phylogenetic niche conservatism under climate change.